1. Field of the Invention
The present invention relates to a new investment material for the pressing loss wax technique of a variety of dental glass ceramics that contain high concentrations of alkaline oxides. Such oxides include lithium oxide, sodium oxide, magnesium and potassium oxide. Also included are other oxides classified as in the flux oxide family such as phosphorous pentoxide, strontium oxide, boron oxide and barium oxide, specifically glass ceramics containing as a final crystalline stage lithium disilicate and lithium monosilicate.
2. Background Discussion
It is well known by those skilled in the art that these glass ceramics when pressed using the loss wax technique, react via the surface of hot glass pressed ceramic and the investment due to its high alkali oxide content. Specifically, reaction occurs when the content of lithium oxide (Li2O) is higher than 12% in weight and there is exposure contact time to high temperature in the range of 800 to 950° C. In the process, the dental glass ceramic ingot is heated above its softening point and then under pressure is forced to fill the empty cavity of the mold made of refractory investment to form the desired shape dental restoration. During this process the surface of the dental glass ceramic in contact with the investment, reacts producing a thick reaction layer, or intermediate layer, caused by the reaction of the free alkali oxides of the glass ceramic with the refractory components of the investment. This reaction layer adheres to the surface of the glass ceramic restoration, and after standard sandblasting, requires additional chemical and mechanical treatments for removing it completely. Normally this undesirable contamination, after standard sandblasting with silica and alumina beads, remains unaffected in the surface of the restoration and still needs to be removed by etching the restoration for about 10 to 15 minutes in a diluted hydrofluoric acid solution. If this contaminated surface reaction layer is not properly removed, it will react with any other porcelain such as the glaze material producing catastrophic failures of the restoration. These failures include, but are not limited to, cracks, rough surfaces, pits, jagged margins and material inclusions that affect shade and aesthetics of the final restoration. Additionally the reaction layer occupies the space that needs to be filled with the dental ceramic and once it is removed, there can be occasional margin fit problems.
By having the ability to press glass ceramic ingots of lithium disilicate such as emax press® (Ivoclar trademark) and lithium silicate such as Obsidian® (Glidewell Laboratories trademark) or any other glass ceramic containing high concentrations of alkali oxides using this new ceramic investment, one can:                1. Provide minimal or no reaction layer formation during the loss wax pressing process.        2. Eliminate the chemical etching process (that otherwise uses extremely corrosive and poisonous hydrofluoric acid) because of the absence of a reaction layer.        3. Press restorations with improved aesthetics, more natural shade and translucency due to elimination of investment inclusions.        4. Provide better surface finish throughout the restoration due to the composition of the investment using nano components with a high surface area.        5. Provide an appropriate investment material with a thermal expansion coefficient similar to the dental glass ceramic pressed to provide a better fit and dimensional shape stability.        
Magnesium phosphate investments have been extensively used as investment for casting alloys and ceramics. Magnesium phosphate investments are produced by the exothermic reaction between magnesium oxide (MgO) and mono amomium phosphate (MAP) in a series of complex reactions that can be summarized as follows:MgO+NH4H2PO4+5H2O→NH4MgPO4.6H2O  1.
The magnesium oxide reacts with mono ammonium phosphate investment incorporating water in the process. Then ammonium magnesium phosphate is heated releasing ammonia and water during the sintering process providing the strength of the material. The chemistry and the effect of water content of the magnesium phosphate investment is well described by Hall et at in the J. Am. Ceram. 81(6), 1550-56 (1998).
Pineda et al in U.S. Pat. No. 6,779,590 describe a phosphate investment composition containing mono ammonium phosphate, magnesium oxide and silica (quartz and cristobalite). They found that controlling the ratio between these three main components has a significant impact on the gas permeability, set time and cast properties. Additionally the particle size distribution of the fillers permits the investment to be burned out rapidly without fracturing. They also describe that the silica filler, which is 72 to 80% of the investment, should have 15-25% of the silica content over 45 microns in size. It is well known to those skilled in the art that these types of phosphate investments using cristobalite and silica as main fillers produce two broad peaks, representing an increase of coefficient of thermal expansion due to the phase transition from alpha to beta in the interval of 200 to 270° C. for the cristobalite and in the interval of 550 to 650° C. for the crystalline silica phase. The appropriate blend of these two fillers produces on average a total increase in the percentage of linear thermal expansion change (PLC) of about 1.5%, sufficient for casting dental ceramics with linear thermal coefficient of expansion (CTE) in the range of 12 to 14×10−6/° C. measured between 25 and 500° C.
Prasad et al. in U.S. Pat. No. 5,180,427 describe a phosphate investment preparation where leucite is added as filler in the range of about 40 to 80% in weight in order to increase the PLC to values greater than 0.84 to 0.87 when heated from 25 to 500° C. The addition of leucite makes the investment suitable for use in dental ceramics with thermal expansion coefficients in the range of 16 to 18×10−6/° C. measured in the interval of 25 to 500° C.
None of the prior art discloses an investment material with specific filler components that help to minimize the undesirable surface reaction layer formed when the glass dental ceramics containing high alkali metal oxides, such as lithium disilicate and lithium monosilicate, are used.